The   I'niversity  of  Washington 
Department  of  Chemistry 


A  Study  of  the  Factors  Influencing  the 

Anodic  Passivity  of  Iron  With  Notes 

on  Polarization  Potentials 


BY 


SETH  CHAPIN  LANGDON 


A  Thesis  Submitted  in  Partial  Fulfilment  of  the  Require- 
ments for  the  Degree  of  Doctor  of  Philosophy 


SEATTLE,  WASHINGTON: 

H.  C.  PIGOTT  PRINTING  CONCERN 

1915 


The  University  of  Washiijgton 
Department  of  Chemistry 


A  Study  of  the  Factors  Influencing  the 

Anodic  Passivity  of  Iron  With  Notes 

on  Polarization  Potentials 


BY 


SETH  CHAPIN  LANGDON 

/i 


A  Thesis  Submitted  in  Partial  Fulfilment  of  the  Require- 
ments for  the  Degree  of  Doctor  of  Philosophy 


SEATTLE,  WASHINGTON: 

H.  C.  PIGOTT  PRINTING  CONCERN 

1915 


TO 
MY   BROTHER 

— S.  C.  L 


323445 


A   STUDY   OF   THE  FACTORS   INFLUENCING   THE 

ANODIC  PASSIVITY  OF  IRON  WITH  NOTES 

ON  POLARIZATION  POTENTIALS. 

Since  the  discovery  of  the  passivity  of  iron  by  James  Keir*1  in  1790, 
a  very  great  number  of  investigations  have  been  devoted  to  it  and  allied 
phenomena.  The  literature  up  to  the  last  six  or  seven  years  has  been 
reviewed  both  by  Byers2  and  by  Heathcote3.  Most  of  the  more  recent 
work  is  summarized  in  the  symposium,  aThe  Passivity  of  Metals,"  pub- 
lished by  the  Faraday  Society  as  a  report  of  a  meeting  held  in  London, 
jS^ovember  12,  1913.  This  session  was  attended  by  many  of  those  in- 
terested in  passivity,  who  indulged  in  a  general  discussion  of  the  subject 
both  from  the  experimental  and  theoretical  sides.  The  appended  biblio- 
graphyf  covers  the  contributions  not  included  in  the  above  mentioned 
summaries. 

A  discussion  of  some  of  the  facteof  passivity  and  of  the  related 
theoretical  considerations,  will  be  found  at  the  close  of  the  experimental 
part.  Meanwhile,  it  will  suffice  to  take  up  the  development  which  led 
to  the  investigations  herein  to  be  detailed.  In  the  review  by  Byers 


f  Lebedev,  Zeit.  Elektrochem.,  18,  891. 

Byers  and  Vorhis,  J.   Am.   Ch.   Coc.,  34,   1368. 

MacLeod-Brown,    Chem.    News,    107,    15. 

F.  Flade  and  H.  Koch,  Z.  Electrochem.,  18,  335-8.  J.  Soc.  Ch.  Ind., 
31,  493. 

N.    Isgarisheb,    Z.    Electrochem,    19,    491-498. 

Grube,  Z.  Electrochem.,  18,  189-211.    J.  Ch.  Soc.,  102,  II,  424. 

Dunstan  &  Hill,  J.  Chem.  Soc.  99,  1853,  Proc.  Chem.  Soc.  27,  222. 

Grave,   Zeit  physik.   Chem.,   77,   513. 

V.  A.  Kistayakoskii,  J.  Chem.  Soc.  100,  II,  401.  Seventh  Interna- 
tional Congress  Applied  Chemistry,  1909  (Sec.  X)  56. 

J.  A.  N.  Friend,  J.  Ch.  Soc.,  101,  50-6   Proc.,  26,  311. 

Flade,  Z.   Physik.   Chem.    76,   513. 

Muller,  W.  J.,  Z.  Electrochem.,  15,  696.     Zeit.  physik.  Chem.,  69,  460. 

P.  Krassa,  Z.   Electrochem.,   14,   607. 

Rudolph   Ruer,   Z.    Electrochem.,    14,    633. 

Trans.  Faraday  Society,  1914.     Symposium  "The  Passivity  of  Metals. 

Armstrong,  Soc.  Chem.  Ind.,  32,  391. 

Smits,  Verslag.     K.  Skad.    Weterschappen,  21,  1132. 

Brunet,  Rev.  gen.  chem.,  17,  66. 

Fr.   Flade  and   H.   Koch,    88,    307. 


*  The  numbers  after  author's  name  refer  to  a  reference  list  at  the  close 
of  the   paper. 


it  was  pointed  out  that  none  of  the  numerous  suggestions  as  to  the 
cause  of  passivity  were  satisfactory,  and  an  attempt  was  made  to  de- 
fine the  term  which  by  reason  of  extension  to  similar  phenomena  had 
become  confused.  The  term  was  defined  as  properly  applied  to  an 
element  when  it  shows  "Abnormal  electro  chemical  relations  and  a 
chemical  inactivity  not  corresponding  to  its  position  in  the  electro 
motive  series  of  the  elements."  It  was  here  also  stated,  that  in  oxygen 
electrolytes  the  occurence  of  the  passivity  in  iron,  used  as  an  anode, 
depended  upon  the  condition  and  previous  treatment  of  the  iron,  the 
nature  and  concentration  of  the  electrolyte,  the  temperature,  the  cur- 
rent density,  and  the  time  of  flow  of  the  current. 

In  a  paper  by  Byers  and  Darrin4  in  1910  it  was  shown,  with 
various  electrolytes,  that  when  the  cell  containing  an  iron  anode  was 
placed  in  a  strong  magnetic  field  the  current  density  required  to  render 
the  iron  passive  was  increased,  that  is,  in  a  magnetic  field  an  iron 
anode  was  more  difficulty  passivified.  In  .a  contribution  published 
one  year  later  by  B[yers  and  Morgan5,  the  same  was  shown  to  be  true 
for  nickel  anodes.  In  the  above  outlined  work  the  results  failed  to 
check  accurately,  and  this  was  attributed  to  the  rise  of  temperature 
due  to  heating  of  the  magnet  or  to  mechanical  difficulties  due  to  the 
form  of  apparatus.  Also  it  was  suggested  that  perhaps  other  un- 
known variables  prevented  exact  duplication  of  results,  so  that  the 
work,  while  it  clearly  showed  the  trend  of  facts,  was  qualitative  in 
its  nature. 

This  research  was  primarily  undertaken  to  verify  the  work  previ- 
ously done  and  since  a  more  powerful  magnet  with  a  water  cooling 
device,  was  available,  it  was  expected  that  more  uniform  results  could 
be  obtained.  Also  this  subject  was  considered  of  particular  interest 
because  of  the  apparent  connection  between  a  chemical  reaction  and 
the  influence  of  the  magnetic  field.  A  search  of  the  literature  had 
revealed  not  a  single  authentic  case  where  any  direct  chemical  effect 
could  be  demonstrated  to  be  due  to  a  magnetic  field.  A  bibliography 
of  the  literature  on  this  subject  is  to  be  found  in  a  paper  published  by 
Byers  and  Langdon  in  19136. 

The  prosecution  of  the  work  led  naturally  to  certain  developments 
connected  with  passivity  in  its  relations  to  gases  dissolved  in  the  elec- 
trolyte and  to  the  potentials  developed  by  iron  anodes.  The  experi- 
mental results  are  therefore  detailed  under  three  heads : 

I.  The  Magnetic  Field  as  Related  to  Passivity,  II.  Dissovled 
Oxygen  as  Related  to  Passivity,  III.  The  Bearing  of  Electrode  Po- 
tential on  Passivity. 


I.      THE  MAGNETIC  FIELD  AS  RELATED  TO  PASSIVITY. 


ELECTRO    MAGNET 


Fig.  i 


The  arrangement  of  the  apparatus  used  in  the  first  series  of  ex- 
periments  is   shown   in   Fig.    1,    and   consisted   of   a    short  test  tube, 

containing  the  electrolyte,  0.5  N  solu- 
tion of  sulfuric  acid,  supported  be- 
tween the  poles  of  a  Weiss  magnet 
capable  of  furnishing  36000  gauss. 
The  anode  was  a  piece  of  iron  wire, 
diameter  0.72  mm.,  sealed  in  a  glass 
tube  with  paraffin  so  that  exactly  4  mm. 
was  exposed.  The  cathode  was  a  plat- 
inum wire  exposing  an  equal  area.  The 
electrodes  were  connected  in  a  series 
with  five  U.  S.  storage  batteries,  a  re- 
sistance box  with  a  range  from  0.1  to 
1000  ohms  and  a  Weston  milli ammeter. 
A  voltmeter,  as  a  shunt,  with  a  key 
connected  the  electrodes. 

At  the  beginning  of  each  experiment, 
iron  anode  was  active.  As  rapidly  as 
possible  the  resistance  was  diminished 
and  the  current  required  for  the  change  to  the  passive  condition  deter- 
mined. The  change  is  characterized  by  a  sharp  rise  in  the  potential 
difference,  as  shown 
by  the  voltmeter,  a  £ 
change  in  the  appear-  § 
aiice  of  the  iron,  an  ? 
evolution  of  oxygen,  = 
and  a  drop  in  the  cur-  c 
rent  shown  on  the  mil-  ° 
liameter.  The  acriti-  < 
cal"  current  noted  E 
in  the  accompanying  ° 
Table  I  is  that  which 
produces  these  changes 
within  ten  seconds  af- 
ter the  imposition  of  a  given  current.  These  results  are  given  for  vari- 
ous samples  of  wire,  numbered  separately,  and  with  various  strengths 
of  magnetic  field.  The  results  as  tabulated  show  clearly  the  rise  of 
"critical"  current  with  field  strength  and  the  general  character  of 
the  curve  is  shown  in  Fig.  2. 


FIELD     STRENGTH  -  GAUSS 
Fig.  2. 


TABLE  I. 


No. 

la 
Ib 
3a 
8a 
3b 
Ic 
3c 
5e 
7e 
9d 
lOa 
lOc 
3e 
6d 
6e 
4c 
4d 
lOd 
2d 
4e 
2e 
5a 
6a 
6b 
6c 
9a 
9b 
9c 
lla 
lib 
lie 
5b 
7d 
8d 
7a 
7b 
7c 


Critical  current, 
M.  amps. 

20 
24 
21 
22 
33 
43 
36 
36 
43 
41 
42 
42 
43 
45 
44 
43 
43 
47 
45 
48 
49 
45 
49 
52 
53 
49 
53 
54 
53 
49 
50 
54 
53 
53 
48 
58 
58 


Field  strength, 
Gauss. 

0 
0 
0 
0 

2700 

4100 

4100 

4100 

5500 

5500 

5500 

5500 

6800 

6800 

6800 

8000 

8000 

8000 

9200 

9200 
10300 
10300 
10300 
10300 
10300 
11200 
11200 
11200 
11200 
11200 
11200 
12000 
13200 
13200 
15200 
15200 
15200 


During  the  progress  of  these  experiments  it  was  noted  that  stir- 
ring of  the  electrolyte  was  produced  when  in  the  magnetic  field  and 
without  carrying  the  field  strengths  to  a  greater  intensity  the  effort 
was  made  to  determine  if  this  agitation  of  the  electrolyte  by  the  field 
had  any  bearing  on  the  delay  in  the  establishment  of  the  passive  con- 
dition. This  is  important  also  because  of  the  extremely  divergent 
views  of  Fredenhagen7  and  of  Grave8.  The  former,  supported  by 
many  of  the  investigators,  viewing  the  establishment  of  passivity  as 
coincident  with  saturation  of  iron  with  occluded  oxygen,  the  latter  as 
simultaneous  with  complete  removal  of  occluded  hydrogen.  It  was 
thought  that  perhaps  stirring  the  electrolyte  with  various  gases  would 
throw  light  upon  both  of  these  questions. 


9 

The  results  were  of  a  character  which  led  to  the  tentative  con- 
clusion that  the  effects  shown  by  the  magnetic  field  were  wholly  due 
to  the  stirring  set  up  thereby  and  consequently  the  stirring  experi- 
ments were  extended  to  the  use  of  a  water  motor,  giving  a  slow  me- 
chanical stirring,  and  an  electric  motor  giving  a  rapid  motion.  The 
stirring  with  gases  was  produced  by  delivering  the  gas  into  the  elec- 
trolyte, from  below  and  on  both  sides  of  the  anode  through  a  double 
nozzle  jet. 

Hydrogen  from  a  Kipp  generator  was  supplied  at  a  low  pressure 
with  consequent  moderate  stirring  effect.  The  air  and  also  nitrogen  were 
furnished  under  small  hydrostatic  pressure,  which  was  not  the  same 
in  the  two  different  gases,  and  the  oxygen  from  a  pressure  cylinder. 
The  violence  of  the  stirring  was,  therefore,  greatest  with  oxygen. 
The  motor  stirring  was  produced  by  using  a  rotating  anode  and  to 
determine  if  the  type  of  motion  were  important  the  motor  was  geared 
with  an  eccentric  so  as  to  produce  a  rapid  perpendicular  motion  of 
the  anode.  The  results  of  these  various  stirring  effects  are  given  in 
tabular  form  in  Table  II  and  for  comparison  the  effect  of  a  magnetic 


TABLE  II. 
1  2345678 

Elec-  Ec- 

H2  N2  Air.  O2          Water        trie          centric 

No  stirring. 

No  field.  F.  P.  F.  F.~^      F.  Motor  F.Motor.  F?'"  F. 

20  42  43  57  48  60  50  67     63     67     55     70      93      95    112  117 
22  48  42  62  48  68  58  72     76     77     61     96    104      83      97  95 

21  47.5  41  58  56  70  67  70     77     68     53     92      85      97      97  88 
20  40  54  67  57  63  68  60     80     58     55     75      97    102      91  93 


Mean:    21     44        45     61     53     65     61     70     74     74     56     83      95      94      99      98 

;     •( 

field  of  12,000  gauss  intensity  without  stirring  and  coincident  with 
stirring  is  given.  Also  in  Column  1  the  critical  current  without  the 
field  is  given.  The  right  hand  set  of  figures  in  each  column  shows 
the  critical  current  in  the  field. 

It  will  be  noted  that  the  magnetic  field  retards  the  establish- 
ment of  the  passivity,  as  indicated  by  the  larger  critical  densities  shown 
under  F  in  column  1.  Agitation  of  the  electrolyte  also  produces  re- 
tardation as  shown  in  the  left  hand  columns  in  2  to  8  inclusive,  and 
the  more  violent  and  effective  stirring  the  greater  the  retardation.  It 
will  be  noted  also  that  the  field  increased  tbf*  Affect  of  the  agitations  of 


10 

lesser  violence  but  the  increased  violence  of  stirring,  the  "field  stirring" 
produces  a  decreasingly  additional  effect  until  with  the  electric  motor 
and  eccentric  no  influence  of  the  field  is  apparent.  It  seems  to  be  con- 
clusive therefore  that  the  magnetic  field  offers  resistance  to  the  estab- 
lishment of  passivity  simply  because  of  the  stirring  effect  which  it  pro- 
duces upon  the  electrolyte  when  undergoing  electrolysis,  which  stirring 
is  visible  in  a  strong  magnetic  field  through  the  violent  agitation  of  the 
bubbles  of  liberated  gases. 

The  stirring  of  an  electrolyte  during  electrolysis  was  first  noted 
by  Hurnuzescu9  in  1895.  Ashcroft10  in  1905  obtaind  a  patent  upon 
stirring  fused  electrolytes  by  a  magnetic  field.  In  1907,  Frary11 
made  application  of  Ashcroft' s  method  to  the  rapid  deposition  of  metals 
by  electrolysis,  substituting  the  magnetic  field  for  rotation. 

As  additional  evidence  bearing  upon  this  conclusion  the  rate  of 
rotation  of  the  anode  was  measured  and  the  corresponding  critical 
current  determined.  The  results  are  given  in  Table  III. 

Here  again  as  in  Table  I  the  numbers  refer  to  a  given  anode  and 
the  letters  to  the  different  experiments  with  each.  These  data  are 
graphically  represented  in  Fig.  3.  The  variable  results  obtained  are 


REVOLUTIONS     PER     MINUTE 


Fig.  3- 


not  important  as  bearing  upon  the  conclusion  because  of  the  factors 
which  are  not  under  exact  control.  It  will  be  evident  that  the,  stirring 
effect  of  a  magnetic  field  will  vary  with  the  position  of  the  anode 
in*  the  field  and  with  any  deviation  from  the  perpendicular,  which 
interferes  with  symmetrical  stirring.  The  experiments  were  all  at 
room  temperature,  which  of  course,  is  variable  to  some  extent  The 
wire  anodes  used,  when  partially  dissolved  were  flattened  on  the  sides, 
so  that  the  resulting  sharp  edges  were  in  the  direction  of  the  lines  of 
force  of  the  field.  In  the  stirring  by  motor  the  results  vary  also  with- 
in limits  and  these  variations  are  to  be  ascribed  not  only  to  changes 


A  <&Tvdj  fi±7tit   frCTtt^       WHtMMCr     ff    G*f'4  ft 


11 

TABLE  III. 

No.  Critical  current                          R.  P.  M. 

la  Table  I                      20  0 

Ib  Table  I                      24  0 

3a  Table  I                      21  0 

8a  Table  I                      22  0 

4d  32  330 

2a  26  400 

2b  27  400 

8b  38  420 

8a  30  420 

8c  38  420 

9a  38  420 

Id  28  420 

Ic  32  480 

7c  33  510 

9b  33  510 

4c  33  540 

8d  33  540 

la  37  600 

Ib  37  600 

3b  50  600 

9c  36  600 

9d  38  600 

4b  41  660 

3c  44  800 

lOd  44  840 

6a  39  840 

2c  34  900 

2d  36  900 

6b  42  980 

6c  44  990 

3a  56  1000 

lOa  50  1020 

lOb  50  1020 

lla  55  1040 

7a  47  1080 

7b  47  1080 

6d  46  1200 

5a  46  1400 

lOc  60  1500 

2e  50  1800 

4a  61  2100 

lib  75  2400 

lie  89  3000 

lid  100  4000 

in  the  rate  during  a  given  experiment  but  the  stirring  by  a  given  rate 
of  rotation  also  depends  upon  variation  from  exact  axial  rotation. 

A  comparison  of  Figures  2  and  3  shows  the  approximate  ratio 
between  the  effect  produced  by  the  field  and  that  due  to  rotation  of 
the  anode,  to  be  about  ten  to  one,  that  is,  ten  gauss  produced  ,the  same 
effect  as  one  revolution  per  minute.  This  ratio  would  of  course  vary 
with  the  form  of  the  apparatus. 


12 

Explanation  of  the  cause  of  stirring  of  an  electrolyte  by  the  mag- 
netic field  are  offered  by  Drude  and  by  TJrbasch13  but  it  does  not  seem 
to  be  anything  more  complicated  than  the  tendency  of  the  movable 
part  of  an  electric  circuit  to  enclose  the  greatest  possible  number  of 
lines  of  force  and  since  the  circuit  is  closed  through  a  liquid  the  mo- 
tion is  the  tendency  of  the  liquid  part  of  the  circuit  to  adjust  itself  to 
the  field  in  accordance  with  this  well  established  principle. 

The  retardation  of  the  passive  state  by  the  field  appears  to  be 
independent  of  the  position  of  the  anode  with  respect  to  the  lines  of 
force.  This  is  shown  by  the  results.,  obtained  in  a  field  of  12,000  gauss, 
given  in  Table  IV. 

TABLE  IV. 

Anode  perpendicular  Anode  parallel  to 

to  lines  of  force.  lines  of  force. 

Critical  current.  Critical  current. 

1  42  m.  amp.  44  m.  amp. 

2  48  m.  amp.  40  m.  amp. 

3  47m.  amp.  46m.  amp. 

4  40  m.  amp.  46  m.  amp. 


Mean   44.3  43.5 

The  result  seems  striking  in  view  of  Kemsen's  observation  that 
a  difference  of  electromotive  force  is  established  between  two  iron  rods 
in  a  magnetic  field  when  parallel  but  not  when  perpendicular  to  the 
lines  of  force. 

The  previous  observations2  that  the  passivity  once  established  is 
maintained  by  a  minimal  current  that  the  passivity  is  not  destroyed 
by  a  magnetic  field4  are  confirmed.  For  example  iron  passive  in  0.5  N 
sulfuric  acid  solution  and  kept  in  that  condition  by  a  current  of  4.5 
milliamperes  remains  passive  when  placed  in  a  field  of  upwards  of 
20,000  gauss.  This  was  unexpected  in  view  of  the  results  recorded 
by  Nichols  and  Franklin14. 

These  experiments  then  show  that  the  magnetic:  field  retards 
the  establishment  of  the  passive  state  in  iron  and  nickel  anodes  be- 
cause of  the  stirring  produced  by  the  field  in  the  liquids  undergoing 
electrolysis.  The  effect  is  exactly  duplicated  by  mechanical  stirring 
produced  either  by  gases  bubbled  through  the  liquid  or  by  motion  of 
the  electrode  itself.  Thus  a  sixth  factor,  that  of  motion  of  the  elec- 
trolyte may  be  added  to  the  list,  already  given,  of  conditions  that  in- 
fluence the  establishment  of  the  anodic  passivity  of  iron. 

The  variation  of  results,  as  clearly  shown  in  Figures  2  and  3  in- 
dicated that,  as  yet,  the  results  obtained  were  qualitative  in  character, 


13 

but  it  was  thought  that  perhaps  the  list  of  variables  was  complete  and 
that  light  could  be  thrown  on  the  subject  by  experiments  in  which 
two  factors  were  allowed  to  vary  mutually  while  the  other  four  were 
held  constant.  Below  is  given,  briefly,  the  experimental  methods  em- 
ployed and  typical  results  obtained  when  the  attempt  was  made  to 
determine  the  relations  between  current  density  and  the  time  required 
to  induce  passivity.  It  was  found  that  it  was  not  necessary  to  use  a 
mechanical  device  to  produce  uniform  stirring  for,  by  the  method  of 
uniform  procedure,  as  pointed  out  below,  the  stirring  effect  introduced 
was  practically  the  same  in  all  cases  and  so  could  be  neglected.  The 
apparatus  used  was  similar  to  that  shown  in  Figure  I  except  that  the 
magnet  was  not  used  and  the  length  of  iron  wire  exposed  was  10  mm. 
instead  of  5  mm. 

The  experimental  procedure  is  outlined  below.  The  external 
resistance  was  first  adjusted  so  that  a  current  of  the  required  density 
would  flow  through  the  circuit,  and  the  time  required  to  convert  an 
active  anode  to  the  passive  condition  was  determined  by  a  stop-watch, 
the  time  recorded  being  the  interval  between  the  closing  of  the  key  and 
the  appearance  of  the  phenomena  previously  described.  If  the  cur- 
rent was  not  too  small,  the  time  required  was  fairly  small,  and  there- 
fore,  iron  was  dissolved  to  an  extent  insufficient  to  materially  affect 
the  diameter  of  the  wire.  It  was,  therefore,  possible  to  make  a  series 
of  determinations  on  one  section  of  wire.  In  order  that  these  should 
be  comparable  the  circuit  was  broken  as  soon  as  the  iron  had  become 
passive  and  the  cell  was  given  a  vigorous  shake.  Then  after  thirty 
seconds  the  circuit  was  closed  again  for  the  next  determination.  This 
interval  renders  the  effect  of  stirring  uniform  and  is  quite  sufficient, 
for  in  0.2  sulfuric  acid  iron,  passive  as  an  anode,  almost  instantly 
becomes  active  when  the  current  is  stopped.  The  character  of  the  re- 
sults is  shown  in  Table  V. 

TABLE  V. 
0.2  N  H2SO4;  temperature,  0°;  10  mm.  wire  exposed. 

Milliamperes                                Time  in  seconds  to  Diameter  of 

current.                                    passivify  the  iron.  the  wire. 

30 12  0.709 

30 22  0.707 

30 24  0.705 

30 26  0.703 

30 25  0.797 

30 28  0.695 

30 28  0.693 

30 30  0.687 

30 29  0.685 

30 29  0.680 

30 38  0.675 

30..  31  0.670 


14 


Disrgarding  the  first  determination,  there  is  a  range  of  differ- 
ence from  the  mean  time  of  28  seconds  which  is  not  to  be  accounted 
for  by  the  small  variation  of  current  density  due  to  change  in  the 
surface  of  the  wire  exposed.  A  large  number  of  similar  determina- 
tions were  made,  using  current  densities  ranging  from  15  to  80  milli- 
amperes  with  a  corresponding  time  required  to  passivify  the  iron 
varying  from  21  minutes  to  3  seconds.  All  experiments  showed,  a 
similar  lack  of  agreement  between  individual  measurements.  Sets  of 
determination  were  also  made  at  18°  and  at  25°,  using  the  cells  in  a 
thermostat.  These  gave  even  less  satisfactory  results,  due,  in  part, 
to  the  greater  loss  of  iron  while  rendering  the  anode  passive1,  greater 
current  densities  being  needed  at  higher  temperatures.  These  extended 
experiments  failed  to  show  the  desired  uniformity  of  relation  be- 
ween  two  variables  and  the  suspicion  that  all  the  factors  influencing 
the  passive  condition  were  not  yet  at  hand  grew  more  pronounced. 

II.     DISSOLVED  OXYGEN  AS  RELATED   TO  PASSIVITY. 

During  the  progress  of  the  work  it  was  noted  that  minute  bub- 
bles of  oxygen  occasionally  adhered  to  the  surface  of  the  iron  even 
after  it  became  wholly  active  and  that  in  such  cases  the  metal  became 
passive  more  readily.  Jarring  the  electrode,  or  stirring  the  electro- 
lyte, partially  obviated  this  difficulty,  but  when  minute  bubbles  were 

allowed  to  persist,  the  results  varied  widely. 
This  suggested  that  dissolved  oxygen  in  the 
neighborhood  of  the  electrode  might  be  a 
factor  in  the  problem, 

T'o  test  the  question  an  apparatus,  Fig. 
4,  was  so  arranged  that  the  iron  anode  could 
be  held  close  beneath  an  atmosphere  of  oxygen 
which  was  kept  at  constant  pressure  in  the 
thistle  tube.  By  rotating  the  tube  bearing  the 
anode,  it  could  be  readily  removed  somewhat 
distant  from  this  sustained  concentration  of 
oxygen.  The  results  obtained  (Table  VI) 
indicate  clearly  that  iron  is  much  more  readi- 
ly made  passive  near  the  oxygen  surface. 
When  hydrogen  gas  or  nitrogen  was  substi- 
tuted for  oxygen  no  such  effect  was  produced. 


Fig.  4. 


15 


TABLE  VI. 
N/5  H,SO4;  room  temperature;   5  mm.  iron  wire  exposed. 


No. 
1.  . 

Time  in 
sec.  for 
30  m.  amps, 
to  passivity. 

...    10 

Diam. 
of  wire 
in  mm. 
0.710 

Relation 
of  iron 
to  oxygen 
bubbles. 

Under 

No. 
12 

Time  in 
sec.  for 
30  m.  amps, 
to  passivify. 
20 

Diam. 
of  wire 
in  mm. 
0  702 

Relation 
of  iron 
to  oxygen 
bubbles. 

Under 

2    . 

...    11 

0  709 

Under 

13 

37 

0  700 

Not  under 

3 

16 

0  708 

Under 

14 

17 

0  700 

Under 

4 

17 

0  708 

Under 

15 

23 

0  700 

Under 

5.  . 

...    13 

0.708 

Under 

16    . 

...    24 

0.697 

Under 

6.  . 

...    13 

0.707 

Under 

17 

45 

0.693 

Not  under 

7 

27 

0  706 

Not  under 

18 

19 

0  693 

Under 

8.  . 

...    26 

0.706 

Not  under 

19.  . 

...    47 

0.692 

Not  under 

9 

15 

0  705 

Under 

20 

25 

0  688 

Under 

10.  . 

...    22 

0.705 

Under 

21 

63 

0.688 

Not  under 

11 

.    22 

0.704 

Under 

22.  . 

.    30 

0.680 

Under 

Nos.   3-6  show  an  average  a  little  above  15  seconds. 

Nos.   7  and  8  not  under  the  oxygen  required  about  twice  the  time. 

No.  9  under  the  oxygen,  goes  down  to  15  seconds  again. 

After  No.  9,  the  iron  was  removed  from  the  electrolyte,  washed  and  dried 
and  the  experiment  discontinued  for  about  20-25  minutes  and  after  this  more 
time  was  required  to  render  the  iron  passive  but  in  general  the  time  required 
under  the  oxygen  was  about  half  that  required  when  not  directly  under  the 
bubble  of  oxygen. 

The  higher  concentrations  of  oxygen  were  next  investigated  with 
the  apparatus  arranged  as  in  Fig.  5.     A  tank  of  oxygen  was  connected 

to  a  manometer  and  thence  to  the 
cell,  which  was  closed  by  a  rub- 
her  stopper  held  in  place  by  a  pres- 
sure clamp.  The  anode  and  cath- 
ode were  arranged  as  before,  ex- 
cept that  the  anode  was  held  in 
place  by  Khotenski's  wax  and 
then  coated  with  paraffin.  A 
length  of  5  mm.  was  exposed.  The 
electrical  connections  were  as  be- 
fore. The  cell  was  placed  in  an 
ice  bath  not  shown  in  the  cut.  All 
factors  shown  to  influence  to  pas- 
sivity were  thus  held  constant  ex- 
cept the  concentration  of  the  oxy- 


To  Pressure  Gay? 

II 


To  Oxygen 
Pressure  Tank 


Variable     Resistance 

Pig.  5. 


gen  above,  and  consequently  in  the 

solution.  After  a  desired  oxygen  pressure  had  been  established  and 
sufficient  time  to  secure  saturation  had  elapsed,  the  cell  was  shaken  with 
a  circular  motion  to  insure  freedom  from  anode  bubbles.  After  thirty 
seconds  the  circuit  was  closed  and  the  time  required  for  a  current  of 
15  milli  amperes  to  passivify  the  iron  was  measured.  The  circuit  was 


16 


Time  in 
seconds  to          Oxygen 
passivify          pressure 
No.             the  iron.        Ibs.  per  sq.  in. 

1  

?     air  that   is   oxygen, 
3  Ibs. 
40 
45 
45 
45 

2  

3 

4  

5  

6  

20 
21 
22                       15  Ibs. 

20 
20 

7  

8  

9. 

10 

11.  . 

12     . 
11 
12 
10  %                 51  Ibs. 
11 
11 
11 

12  
13  
14  

15  

16    . 

17.  . 

TABLE  VII. 


No. 


Time  in 

seconds  to  Oxygen 

passivify  pressure 

the  iron.  Ibs.  per  sq.  in. 


18.  .  .  . 

7 

19.  .  .  . 

6% 

20.  .     . 

7 

57.4  Ibs. 

21 

6^ 

22 

7 

23 

5 

24 

6 

25.  .  .  . 

5 

65.88  Ibs. 

26.  .  .  . 

5 

27.  .  .  . 

4 

28.  .     . 

..'-      4% 

29 

5 

81  34  Ibs 

30 

4 

31.  .  .  . 

4 

32 

34 

Here  fresh  electro- 

33 

45 

lyte    substituted 

34.  .  .  . 

45 

Saturated      with 

air,  i.  e.,  oxygen, 
3  Ibs. 

0.2  N  sulfuric  acid.  5  mm.  length  of  iron  wire  exposed.  Initial  diameter 
0.710  mm.  Final  diameter  0.680  mm.  15  ma.  current  employed  while 
rendering  iron  passive. 

then  opened  and  measurements  repeated.  Table  VII  shows  a  series 
for  pressures  of  oxygen  ranging  from  3  Ibs.  per  sq.  in.  to  81  Ibs,  with 
one  section  of  wire.  The  values  so  obtained  represent  a  very  satis- 
factory constancy  and  are  typical  of  a  large  number  of  series  which 
are  not  detailed. 

It  will  be  observed  that  loss  of  surface  area,  due  to  solution  of 
the  iron,  is  not  a  seriously  disturbing  factor  since  the  decrease  of 
diameter  of  the  iron  in  successive  determinations  is  about  0.0009  mm., 
and  only  0.03  mm.,  jn  the  whole  series  of  34  determinations.  When 
an  oxygen  pressure  of  0.2  atmosphere,  secured  by  air  pressure  was 
used  no  such  uniform  agreement  in  time,  required  to  render  iron  pas- 
sive, could  be  obtained.  This  accounts  fully  for  the  long-continued 
failure  to  secure  quantitative  results  under  atmospheric  pressures. 

Similar  experiments  were  conducted  using  0.2  N"  sulfuric  acid, 
which  had  been  boiled  to  expel  dissolved  gases,  and  with  the  cell  con- 
nected to  a  suction  pump.  The  results  obtained  showed  great  irregu- 
larity, but  the  time  required  to  render  passive  was  much  longer  than 
under  ordinary  conditions. 
MW  When  0.2  nitric  acid  was  used  as  the  electrolyte,  conditions  be- 


17 

ing  otherwise  the  same  as  those  above  detailed,  the  results  were  dif- 
ferent; the  iron  becomes  active  much  less  readily  in  nitric  acid.  It 
was  finally  found  necessary  to  remove  the  iron  after  each  determina- 
tion, wash  with  0.2  N  sulfuric  acid  and  with  water  before  a  satisfac- 
tory repetition  could  be  obtained.  With  nitric  acid  also,  the  factor 
of  long  continued  passage  of  the  current  is  less  marked  in  its  effect 
than  with  sulfuric  acid,  and  a  critical  density  is  more  apparent.  Be- 
low this  critical  density,  even  long  continued  passage  of  the  current 
only  infrequently  produced  passivity.  If  the  current  density  was  suf- 
ficiently great,  passivity  was  established  practically  instantly.  For 
0.2  "N  nitric  acid  solution  at  atmospheric  pressure,  i.  e.,  about  3  Ibs. 
per  sq'.  in.  of  oxygen  pressure,  the  critical  current  was  found  to  be 
20  milliamperes.  At  56  Ibs.  oxygen  pressure,  the  critical  current  was 
13  milliamperes.  It  will  be  observed  that  increase  of  oxygen  pres- 
sure here  lowers  the  current  density  required  to  render  the  iron  pas- 
sive. 

In  0.01  N  hydrochloric  acid  with  53  Ibs.  oxygen  pressure,  oxy- 
gen was  evolved  freely  and  the  other  phenomena  of  passivity  appeared 
when  the  current  density  was  relatively  high.  When  the  other  con- 
ditions, except  the  pressure  of  gas  imposed,  were  similar,  there  was 
only  a  very  slight  evolution  of  gas  at  the  anode.  With  0.02  !N"  hydro- 
chloric acid  less  oxygen  was  evolved,  and  with  0.05  !N"  and  0.1  N"  solu- 
tions no  indications  of  passivity  either  with  or  without  oxygen  pres- 
sure could  be  observed.  It  appears  that  with  a  sufficient  dilution  of 
chlorine  ions  the  passive  state  can  be  induced  in  halogen  solutions:  a 
fact  not  heretofore  demonstrated. 

From  the  foregoing  results  it  appears  that  to  the  five  factors 
mentioned  in  the  introduction,  which  condition  the  passive  state,  are 
to  be  added  two  others:  The  character  and  extent  of  motion  of  the 
electrolyte  and  the  concentration  of  dissolved  oxygen  about  the  anode. 
When  all  of  these  factors  are  being  taken  into  account,  constant  re- 
sults are  obtainable  with  respect  to  the  time  required  to  render  iron 
passive  with  a  given  current  in  sulfuric  acid,  and  in  nitric  acid  a 
critical  current  density  is  determinable. 

III.     THE    BEARING    OF    ELECTRODE    POTENTIAL    ON 

PASSIVITY. 

The  discovery  that  the  concentration  of  oxygen  dissolved  in 
the  electrolyte  was  a  factor  influencing  the  anodic  passivity  of  iron 
raised  a  question  as  to  its  possible  effect  on  the  electrode  potential  of 
iron.  It  has  been  assumed  by  Fredenhagen,  Muthman,  Fradenberger 
and  others  that  the  anodic  potential  is  a  measure  of  the  passivification. 


18 

In  attempting  to  answer  these  questions  results  were  obtained  which 
were  not  so  directly  related  to  passivity  as  was  expected  when  the  meas- 
urements were  undertaken,  however,  they  were  of  such  interest  as  to  de- 
serve a  detailed  report. 

The  whole  situation  of  electrode  potentials  is  far  from  being 
in  a  satisfactory  condition,  and  due  to  its  importance  and  interest 
should  prove  a  fruitful  field  for  a  thorough  and  systematic  experi- 
mental development.  Approximate  electromotive  values  are  given  by 
most  authors ;  while  for  iron,  as  Heatheote3  has  pointed  out,  the  values 
given  by  the  different  authorities  show  a  surprising  lack  of  agreement. 
In  most  texts  the  potential  value  of  0.940  volts  is  given  for  iron 
in  contact,  at  room  temperature,  with  a  solution  containing  one  gram 
mole  of  iron  ions  per  liter  measured  against  a  normal  calomel  electrode 
as  zero.  Since  all  of  the  studies  on  passivity,  described  in  the  first 
two  parts  of  this  report,  were  made  with  iron  immersed  in  0.2  N" 
sulfuric  acid  and  at  the  temperature  of  an  ice  bath,  these  conditions 
were  adopted  for  use  in  these  potential  measurements.  This  was 
done  so  that  the  two  phases  of  the  work  could  be  more  readily  com- 
pared. 

In  order  to  work  with  high  gas  pressures  the  normal  calomel 
electrode  was  modified  somewhat  in  form.  The  cell,  (See  Fig.  6) 
used  was  kindly  constructed  by  Prof.  Wm.  M.  Dehn  and  differed 
from  the  ordinary  cell  in  the  following  particulars :  the  platinum 
contact  with  mercury  was  sealed  in  through  the  glass  at  the  bottom 
of  the  cell ;  the  tube  for  contact  with  the  electrolyte  bearing  the  metal 
under  investigation  was  drawn  out  to  a  long  and  rather  fine  capil- 
lary; a  dropping  funnel,  for  introducing  the  normal  potassium  chlor- 
ide, was  sealed  on  the  cell  at  the  top.  Thus  it  was  possible  to  keep 
the  cell  always  full  of  the  normal  potassium  chloride  solution,  and 
this  absence  of  an  air  space  prevented  external  pressure  from  forcing 
any  other  electrolyte  back  into  the  body  of  the  cell.  The  capillary 
tube  prevented  a  rapid  diffusion  and  consequent  contamination.  After 
each  set  of  measurements  fresh  normal  potassium  chloride  was  intro- 
duced through  the  dropping  funnel  thus  forcing  that  out  in  the  capil- 
lary which  had  been  in  contact  with  the  outside  electrolyte. 

That  the  value  of  this  cell  did  not  change,  in  the  course  of  the 
six  months  that  it  was  used,  was  shown  by  checking  it  from  time  to 
time  with  two  normal  calomel  electrodes  of  the  ordinary  type  and  the 
values  obtained  for  all  three  did  not  show  any  noticeable  variation. 
Against  amalgamated  zinc,  immersed  in  molor  zinc  sulphate  and  at 
room  temperature,  all  gave  values  varying  but  little  from  1.08  volts, 


19 


Fig.  Q. 


which  is  given  as  the  characteristic  potential  difference  of  this  com- 
bination. The  potentials  measured  are  compared  with  the  normal 
calomel  cell  as  zero  and  the  negative  sign  indicates  potentials  differ- 
ing from  the  calomel  as  do  the  noble  metals. 

A  potentiometer,  capable  of  detecting  potential  differences  of 
one  ten  thousandth  of  a  volt,  was  used,  but  readings  were  taken 
only  to  the  third  decimal,  for  the  potential  fluctuates  from  moment 
to  moment  due  to  a  variety  of  causes  such  as  local  concentration  and 
thermal  effects.  In  numerous  cases,  even  this  degree  of  accuracy  was 
not  practical.  The  instrument  used  was  very  satisfactory  as  it  al- 
lowed of  quick  and  accurate  adjustment.  The  initial  electromotive 
force  of  different  sections  of  wire  immediately  after  immersion  in 
the  sulfuric  acid  was  not  always  the  same,  but  ranged  between  the 
limits  +0.522  and  +0.506  volts.  The  variation  in  the  initial  ac- 
tive potential  is  perhaps  somewhat  related  to  a  similar  variation  in 
the  differences  of  time  required  for  a  given  current  density  to  pas- 


20 

sivify  different  sections  of  wire.  These  small  variations  are  prob- 
ably to  be  ascribed  to  variations  in  character  of  the  electrode  material. 
As  the  iron  stands  in  contact  with  the  acid  its  potential  gradually 
changes  as  shown  in  Table  VIII,  though  the  variations  here  are  of 

TABLE  VIII. 


Time  in 
minutes 
Start 
2 

19 

31 

37 

45 

60 

72 

87 
125 
140 
150 

152 
153 

156 


E.  M.  F. 
Volts 
+0.511 
+0.513 
+0.523 
+0.524 
+0.524 
+0.527 
+0.530 
+0.536 
+0.536 
+0.539 
+0.542 
+  0.544 

Iron  changed  to  fresh  electrolyte 

+0.549 
+0.549 
+0.550 


a  more  uniform  character  and  of  greater  magnitude  than  were  shown 
in  all  cases.  An  exactly  similar  set  of  figures  appear  in  Table  IX, 
where  at  the  end  of  seven  minutes  hydrogen  gas,  at  50  Ibs.  pressure 

TABLE  IX. 
Hydrogen  Pressure 


Time  in 
minutes 
Start 
2 
4 
7 

10 
12 
13 
15 
18 
20 

Start 

4 

7 

8 

9 

13 
18 
19 
20 
25 
27 


No  external  gas  pressure 


Hydrogen   pressure    50    Ibs.    per   sq. 
Pressure  same 


E.  M.  F. 

0.507 
0.510 
0.511 
0.513 
0.515 
0.517 
0.518 
0.518 
0.519 
0.520 
Oxygen  Pressure 

0.51U      No  external  pressure 

u.514 

0.515 

0.517      Oxygen  pressure  53  Ibs.  per  sq.  in. 

0.519      Same  pressure  of  Oxygen 

0.52± 

0.523 

0.524 

0.524 

0.526 

0.527 


in. 


21 

per  sq.  in.,  was  applied.  As  a  comparison  of  Table  VIII  and  IX  will 
show,  the  gas  at  this  pressure  caused  no  noticeable  deviation  from  the 
normal  behavior.  Oxygen  pressure  showed  a  corresponding  lack  of 
effect  (See  Table  IX)  on  prolonged  application,  thus  allowing  the 
electrolyte  to  become  saturated  with  the  gas. 

It  may  therefore  be  concluded  that  neither  mechanical  pressures 
nor  the  saturation  of  the  electrolyte  with  hydrogen  or  oxygen  produces 
an  effect  on  the  electromotive  force  of  iron  when  immersed  in  0.2  X 
sulfuric  acid. 

The  second  step  was  to  obtain  the  electrode  potential  of  iron 
electrodes  held  in  the  passive  condition  by  a  small  current,  and  sub- 
sequently to  determine  the  effect  of  oxygen  and  other  gaseous  pres- 
sures. The  apparatus  is  shown  in  Fig.  6,  and  requires  but  little  ex- 
planation. The  iron  electrode  was  in  series  with  a  platinum  electrode, 
a  milliammeter,  five  lead  storage  batteries,  a  variable  resistance  and 
a  key.  The  potential  difference  between  the  normal  calomel  electrode 
and  the  iron  electrode  could  be  measured  at  any  time  by  the  potentio- 
meter which  was  connected  through  a  commutator. 

With  the  arrangement  just  described,  a  section  of  wire  was 
rendered  passive  at  high  current  density  and  then  the  resistance  of 
the  external  circuit  increased  until  the  polarizing  current  had  dropped 
to  10  milli amperes.  The  first  reading  of  the  potential  just  after  pas- 
sivification  were  always  higher  .than  subsequent  values.  These  changes, 
as  appears  in  Table  X,  show  that  polarized  passive  electrode  becomes 

TABLE   X. 

Time  in 

minutes  E.  M.  F. 

Start  +0.510 

4  +0.512 

7  — 1.790      Iron  passive,  10  M.  A.  flowing 

10  — 1.789 

16  — 1.785 

20  — 1.782 

22  — 1.783 

24  — 1.781 


less  noble  on  standing  even  though  a  small  external  current  is  main- 
tained to  sustain  the  polarization.  This  is  exactly  analagous  to  the 
behavior  of  the  active  iron  when  no  external  current  is  applied.  (Com- 
pare Table  VIII.) 

The  effect  on  this  polarization  potential  produced  by  the  applica- 
tion to  the  surface  of  the  electrolyte,  of  oxygen  gas  under  pressure  is 
shown  in  Table  XI,  where  it  is  clearly  evident  that  coincident  with 


22 

TABLE  XI. 

Time  in 

minutes          E.  M.  F. 

Start  +0.506  No  pressure,  no  external  current 

1  +0.560 

2  +0.507 

Iron  rendered  passive  and  then  current  reduced  to  10  M.  A. 

3-5  ?  10  M.  A.  current.     Potential  varying 

7  — 1.820  "   "      " 

10  — 1.816  "    "      " 

12  — 1.818  "   "      " 

15  "   "      "        "   Oxygen  pressure  42  Ibs.  sq.  in. 

17  —1.806  "   "      "        "  "  "  "      "      "     " 

22  — 1.807  "   "      "        "  "  "  "      "      "     " 

23  "   "     "        "   Pressure  off 
25  — 1.811  "   "      " 

30  — 1.788  "   "      "        "   Oxygen  pressure  42  Ibs.  sq.  in. 

32  — 1.786  "    "      "        "  "  "  "      "      "     " 

33  —1.788  "    "      "        "  "  "  "      "      "     " 
36  — 1.796  "   "      "        "           "        off 

38  —1.798  "    "      " 

39  •    — 1.796  "    "      " 
41  — 1.796  "   "      " 

43  "   "      "        "          "        pressure  48  Ibs.  sq.  in. 

44  — 1.786  "    "      "        "  "  "  "      "      "     " 
46  —1.787  "   "      "        "           M               "  "      "      "     " 
48  — 1.788  "    "      "        "           "               "           "      "      "     " 

53  — 1.793  "   "      "        "  "  "  "      "      "     " 

54  +0.518  No  external  current  or  pressure 


the  rise  in  pressure  there  is  a  fall  in  potential,  and  that  with  the  re- 
lease of  the  pressure  the  potential  again  rises,  but  not  as  high  as  it  was 
when  the  pressure  was  first  applied.  This  is  not  surprising  in  view  of 
the  fact  just  established  in  the  previous  experiment,  that  for  an 
anodically  polarized  passive  iron  electrode  the  value  of  its  potential 
gradually  decreased  with  time. 

That  the  change  in  potential  is  due  to  the  mechanical  pressure 
and  is  independent  of  the  nature  of  the  gas  is  also  made  clear  by  the 
data  in  Table  XII.  A  range  of  oxygen  pressures  was  used  and  then 
a  corresponding  treatment  using  hydrogen  pressure.  The  results  as 
shown  here  are  more  uniform  than  those  exhibited  by  most  electrodes. 
The  measurements  in  the  table  were  all  made  on  one  section  of  iron 
wire  but  the  data  indicated  was  previously  ascertained  on  divers  sec- 
tions and  the  results  as  shown  are  typical  and  are  confirmed  by  numer- 
ous independent  tests.  The  table  shows  the  most  complete  and  satis- 
factory single  series  of  measurements. 

When  this  work  was  undertaken  it  was  expected  that  if  the  gas 
pressure  produced  any  effect  it  would  be  after  the  electrolyte  had  be- 
come saturated  with  the  gas,  and  that  if  the  hydogeii  produced  any  re- 
sult at  all  that  it  would  be  opposite  to  that  caused  by  oxygen.  The 


TABLE  XII. 


Time  in 

minutes 

E.  M.  F. 

Start 

+  0.510 

3 

+  0.510 

5 

+  0.510 

8 

—  1.784 

12 

—1.77] 

13 

—  1.771 

14 

—  1.730 

15 

—1.768 

17 

—1.771 

18 

—  1.730 

19 

—  1.730 

20 

—  1.730 

21 

—  1.768 

23 

—1.730 

25 

—  1.748 

26 

—  1.760 

27 

—  1.770 

28 

—  1.760 

30 

—  1.751 

31 

—  1.740 

32 

—  1.730 

34 

'  —1.767 

35 

+0.520 

38 

+  0.520 

40 

—  1.775 

41 

—  1.776 

44 

—  1.728 

48 

—  1.747 

50 

—  1.755 

51 

—  1.761 

53 

—1.770 

Iron  active  and  no  external  current 

Iron  rendered  passive  then  current  lowered  to  10  M.A. 

Lowered  to  10  M.  A. 

Oxygen  pressure  turned  on  43.9  Ibs. 

Oxygen  pressure  off 

Oxygen  pressure  18.7  Ibs. 

43.9  " 

43.9  " 

43.9     " 
No  pressure 
Oxygen  pressure  43.9  Ibs. 

30.5     " 

25.8  " 
No  pressure 

Oxygen  pressure  25.8  Ibs. 

30.5  " 

34.6  " 

43.9  " 
No  pressure 

External  10  M.  A.  current  stopped 

Iron  active 

Iron  rendered  passive  and  current  lowered  to  10  M.  A. 

Hydrogen  pressure  43.9  Ibs. 

34 

30.5     " 

25.8     " 
No  pressure 


fact  that  the  change  of  potential  was  coincident  with  the  application 
of  the  pressure  and  independent  of  the  gas  led  to  the  suspicion  that 
perhaps  the  phenomena  was  not  characteristic  of  passive  iron  alone.  To 
test  this  potential  determinations  were  made,  under  analagous  con- 
ditions, on  the  following  electrodes;  an  iron  cathode,  platinum  anode 
and  cathode,  and  a  copper  anode  Table  XIII  shows  the  changes  pro- 
duced. The  data  for  cathodes  is  only  approximate,  due  to  lack  of  con- 
stancy, so  that  the  results  are  only  qualitative  although  the  change 
with  pressure  is  quite  evident  and  it  becomes  clear  that  on  application 
of  gaseous  pressure  anodes,  from  which  gas  is  'being  evolved,  become 
more  positive  and  cathodes  more  negative.  Nernst's  equation  for  the 
potential  of  metal  electrodes  can  be  extended  over  to  the  effects  just  de- 
scribed if  the  anode  is  considered  as  coated  with  an  oxygen 
film  or  charged  with  oxygen,  as  by  occlusion.  In  the  equation 

E  =  { nat  log  —    1 ,  P  would  represent  the  solution  tension  of  the 

P  n    <  p     J 

oxygen  which  is  associated  with  the  metal  and  p  would  represent  the 


24 

TABLE  XIII. 

Platinum  Anode  and  Gas  Pressure. 

N/5  H2SO4;  temperature  0°  C.    Polarizing  Current  8  M.  A. 

Time  in 

Hydrogen  pressure 

minutes 

E.  M.  P. 

Ibs.  per  sq.  in. 

Start 

—  1.899 

0 

1 

—1.898 

0 

2 

—  1.879 

50 

3 

—  1.882 

40 

7 

—  1.884 

38 

9 

—1.885 

35 

12 

—  1.890 

28 

13 

—  1.898 

0 

15 

—  1.898 

28 

18 

—1.893 

34 

21 

—  1.886 

46 

24 

—  1.882 

57 

26 

—  1.882 

57 

27 

—1.903 

0 

Iron  Cathode  and  Gas  Pressure 

Time  in 

minutes 

E.  M.  P.               Pressure 

. 

Start 

+0.069      No  pressure 

4 

+  1.069 

7 

+  1.058      Hydrogen  40  Ibs.  per  sq 

.  in. 

9 

+1.057               "           "      " 

10 

+  1.057               "            "      " 

15 

+1.065      No  pressure 

20 

+1.069       " 

Copper  Cathode  and  Gas  Pressure 

Time  in 

minutes 

E.  M.  P.               Pressure 

Start 

+1.141     No  pressure 

-j     . 

2 

+1.140 

4 

+1.127      Oxygen  pressure  29  Ibs. 

per  sq.  in. 

6 

+  1.050      41  Ibs.  per  sq.  in. 

7 

+  1.160      Pressure  off. 

Platinum  Cathode  and  Gas  Pressure 

Time  in 

minutes 

E.  M.  P.               Pressure 

Start 

+1.680      No  pressure 

2 

+0.680       " 

5 

+0.685 

7 

+0.673      Oxygen  pressure  43  Ibs. 

per  sq.  in. 

8 

+0.697     No  pressure 

10 

+0.673      Oxygen  pressure  43  Ibs. 

per  sq.  in. 

11 

+0.676               "             "          "      " 

«         « 

13 

+0.793      No  pressure 

osmotic  pressure  of  oxygen  ions  in  the  solution.  The  application  of 
any  external  pressure  would  then,  immediately  increase  the  concen- 
tration of  the  oxygen  associated  with  the  metal  and  thus  raise  the 
solution  tension  P  and  the  value  of  E  the  potential.  On  the  other  hand, 


25 

the  increase  of  the  value  of  the  osmotic  pressure  p  would  be  a  slow 
process  so  that  on  application  of  external  pressure  the  value  P  in- 
creases faster  than  p  and  there  results  the  rise  in  anode  potential. 
Exactly  analogous  relation  exist  in  the  case  of  the  cathode  and  a  simi- 
lar explanation  of  the  increased  negativity  of  cathodes  can  be  made. 
From  the  form  of  the  factor  (nat  log  P/p)  it  is  to  be  expected  that  the 
change  of  potential  with  pressures,  such  as  those  used,  would  be  small 
and  taking  into  consideration  the  facts  indicating  the  presence  of  an 
oxygen  film  this  is  the  most  useful  hypothesis  to  apply  in  this  case 
where  the  change  in  potential  is  small,  such  as  would  be  expected  if 
a  gas  film  were  considered,  while  if  occluded  oxygen  or  an  oxygen 
charge  is  assumed  we  are  at  a  loss  due  to  lack  of  knowledge  of  the  co- 
efficient of  occlusion.  The  effects  of  pressure  on  potential  as  above 
detailed  are  new  and  merit  fuller  investigation.  The  potential  meas- 
urements led  directly  to  the  studies  which  follow. 

In  the  hope  of  obtaining  a  more  intimate  knowledge  of  the  pro- 
cesses of  passivification  a  study  was  made  of  the  changes  in  the  po- 
tential of  iron  while  it  was  passing  from  the  active  to  the  passive  state, 
and  the  reverse.  The  conditions  and  apparatus  used  were  exactly  the 
same  as  in  the  work  just  detailed.  The  field  of  polarization  potentials 
is  one  of  unusual  difficulty  because  an  electrode  in  contact  with  an 
electrolyte  shows  great  variation  in  behavior,  especially  when  an  ex- 
ternal current  is  impressed.  Using  the  potentiometer  previously  de- 
scribed the  changes  in  potential  when  the  iron  was  used  as  an  anode 
were  found  to  be  interesting,  but  when  attempts  were  made  to  get  ac- 
curate readings  of  the  momentary  values  it  was  found  that  satisfactory 
and  reproducible  values  could  not  be  obtained.  For  the  sake  of  clear- 
ness the  observed  behavior  will  be  described  and  then  an  account  will 
be  given  of  the  method  used  in  obtaining  the  results.  The  potential 
behavior  of  iron  in  contact  with  fifth  normal  sulphuric  acid  when  no 
external  current  is  flowing  has  been  previously  described,  and  as  was 
shown  in  Table  VIII,  the  potential  gradually  became  more  positive 
on  standing.  'This  value  does  not  attain  constancy  in  the  course  of 
two  hours.  Different  specimens  of  iron  wire  on  immersion  in  the  acid 
show  potentials  ranging  from  +0.506  to  +0.522  volts.  When  the  ex- 
ternal current,  10  m.  amps.,  was  applied  the  potential  quickly  became 
less  positive  and  by  the  close  of  two  seconds  it  had  generally  passed 
through  the  value  +0.11  volts  (See  Fig.  7).  Then  the  change  became 
slower  and  there  was  a  continuous  fall  to  about  —0.15,  and  this  was 
soon  followed  by  a  very  rapid  fall  to  the  neighborhood  of  — 1.85. 
Coincident  with  this  last  sudden  drop  the  iron  became  passive  and 


26 


-ISO 


Fig.  7. 


the  resistance  at  the  anode  surface  increased  as  was  shown  by  the  fall 
in  polarizing  current. 

When  the  external  current  was  stopped  the  potential  did  not 
change  greatly  for  a  little  while  and  then  there  was  a  very  sudden  rise 
as  the  iron  became  active.  The  wait  before  activification  sets  in  is 
longer  when  the  passive  electrode  has  been  polarized  for  a  greater 
length  of  time  or  when  a  high  current  has  been  used.  On  becoming 
active,  with  no  external  current  flowing,  the  potential  usually  rose  to 
a  value  slightly  above  0.6  volts,  which  is  higher  than  the  initial  value  on 
first  immersing  the  iron  in  the  electrolyte.  Potentiometer  readings 


27 

could  not  be  made  in  the  ordinary  way  since  during  the  processes,  the 
values  were  constantly  changing.  The  current  through  the  external 
circuit  was  changing  in  an  analagous  manner  and  this  tends  to  show 
that  the  change  of  potential  is  associated  with  a  change  in  resistance 
at  the  electrode  surface.  The  external  resistance  was  so  arranged  that 
there  was,  as  nearly  as  possible  a  current  of  10  milli-amperes  flowing 
during  the  period  shown  on  the  curve  by  the  length  B  to  G.  Using 
these  conditions  the  method  adopted  was  to  choose  a  time,  say  five 
seconds,  after  the  external  current  was  applied  and  then  attempt  to 
get  as  good  readings  of  the  potential  at  this  time  as  possible.  The 
potentiometer  circuit  was  closed  with  the  instrument  set  at  the  esti- 
mated potential,  then  by  the  vigor  and  direction  of  the  galvanometer 
throw  the  setting  was  guessed  and  after  the  iron  had  been  rendered 
active  again  another  trial  was  made  at  the  end  of  the  fifth  second. 
Thus  a  value  was  approached,  but  there  was  always  the  difficulty  that 
the  potential  of  any  particular  specimen  of  wire  was  always  changing 
and  different  samples  gave  different  values  so  that  the  exact  quantita- 
tive results  were  impossible.  However,  the  graph  gives  a  very  good 
idea  of  the  trend  of  events  and  the  failure  to  obtain  concordant 
results  is  characteristic  of  work  in  this  field.  Heathcote3  at- 
tempted a  similar  set  with  even  less  success  although  he  arrived  at  a 
similar  curve  for  the  changes  of  potential  as  iron  becomes  passive.  The 
form  of  the  curve  (See  Fig.  7)  is  such  that  it  presents  rather  strong* 
evidence  in  favor  of  the  idea  that  the  establishment  of  passivity  is  the 
result  of  two  processes.  The  first  of  these  would  consist  of  the  forma- 
tion of  a  layer  of  insoluble  material  such  as  ferrous  oxide.  This  would 
not  be,  of  necessity,  a  firmly  adherent  coating  and  might  be  pierced  by 
holes  or  pores.  Its  initial  formation  would  correspond  to  the  part  of 

the  curve  from  A  to  B.  The 
other  process  would  be  the  es- 
tablishment of  a  film  of  oxygen 
gas  which  would  form  a  com- 
plete cover  over  the  electrode. 
The  transition  between  these  two 
processes  is  represented  by  the 
slow  drop  in  potential  shown  be- 
tween B  and  G,  after  which  the 
cover  rapidly  becomes  complete 
and  potential  drops  from  G  to 
D.  In  a  similar  way  the  curve 
indicating  that  activification  is  a 
single  process  or  at  best  prac- 


28 

tically  the  simultaneous  removal  of  both  the  oxygen  and  the  oxide 
coats.  This  removal  is  exceptionally  complete  as  is  shown  by  the  final 
maximum  potential.  This  may  also  be  due  to  the  removal  of  the  oc- 
cluded gases. 

That  the  variation  of  anode  potential  is  associated  with -a  change 
of  resistance  at  the  anode  surface  is  confirmed  by  the  change  in  the 
polarizing  current  as  the  iron  becomes  passive.  The  graph,  (See  Fig.  /* 
8)  shows  that  the  variation  in  external  current  with  time  is  exactly 
analagous  to  the  changes  in  the  anode  potential  during  the  process  of 
passivification.  In  taking  the  date  (Table  XIV)  a  longer  section  of 

TABLE  XIV. 

Time  in  Current, 

seconds  Milli.  Amps. 

0  59  Conditions 

3  58  Temperature   18°   C. 

10  57  0.2  N  H2SO4 

17  56M$ 

32  &6*4  Diameter  of  iron  wire  0.770  mm. 

75  56  25  mm.  wire  exposed. 

85  56  The    wire    had     previously    been 

95  55^  rendered  passive  two  or  three  times. 

105  55  Several     successive     determinations 

110  54  on    the   same   section    of  wire   gave 

117  53  exactly    similar    results.      Through- 

120  46^  out  the  work  the  external  resistance 

was  not  changed. 

iron  was  used  in  order  to  obtain  figures  of  greater  magnitude.  The 
bearing  of  these  curves  on  the  explanation  of  passivity  will  be  given  in 
the  theoretical  discussion  at  the  close  of  the  paper.  As  a  preliminary 
some  of  the  facts  that  are  considered  of  critical  importance  will  be 
given,  followed  by  a  review  of  the  theories  that  are  now  exerting  the 
greatest  influence.  Since  the  historical  development  has  been  well 
treated  in  the  literature  a  number  of  times,  space  will  be  used  here 
simply  to  present  the  facts  and  theories. 

The  isolated  facts  of  passivity  are  so  numerous  and  so  varied 
that  for  the  purpose  of  this  discussion  it  will  be  sufficient  to  men- 
tion only  a  few  of  the  more  important  and  those  which  bear  most 
directly  on  the  theoretical  considerations  that  will  follow.  The  sum- 
maries by  Byers2  and  by  Heathcote3  as  well  as  the  Faraday  Societ}^ 
Symposium15  to  which  reference  has  been  previously  made,  were  free- 
ly consulted  and  due  credit  is  given  to  these  reviews  where  the  facts 
are  more  completely  detailed  and  the  various  authors  and  references 
are  given. 

The  discovery  of  passivity  is  accredited  to  James  Keir1,  who 
in  1790  investigated  the  observation  of  Bergman16  that  some  samples 


29 

of  iron  will  displace  silver  from  its  salts  while  others  will  not.  As  an 
introduction  to  the  phenomena  of  passivity  a  quotation  of  Keir's  re- 
port, of  one  of  his  most  important  observations  will  be  valuable.  H'e 
states,  "I  digested  a  piece  of  fine  silver  wire  in  pure  and  pale  nitrous 
(nitric.)  acid,  and  while  the  solution  was  going  on  and  before  satura- 
tion was  completed  I  poured  a  portion  of  the  solution  upon  pieces  of 
clean  and  newly  scraped  iron  wire  into  a  wine  glass  and  observed  a 
sudden  and  copious  precipitation  of  silver.  The  precipitate  was  at 
first  black,  then  it  assumed  the  appearance  of  silver  and  was  five  to 
six  times  larger  in  diameter  than  the  piece  of  iron  wire  it  enveloped. 
The  action  of  the  acid  on  the  iron  continued  some  little  time  and  then 
ceased,  the  silver  redissolved,  the  liquor  became  clear,  and  the  iron  re- 
mained bright  and  undisturbed  in  the  bottom  of  the  wine  glass,  where 
it  continued  during  several  weeks  without  suffering  any  change  or 
effecting  any  precipitation  of  the  silver." 

He  noted  that  the  passive  iron  became  active  on  being  scratched 
or  jarred  or  on  contact  with  a  piece  of  active  iron.  Indeed  he  re^ 
ported  that  if  a  piece  of  passive  iron  was  placed  close  to,  but  not  touch- 
ing, a  sample  of  passive  iron  which  lay  in  an  acid  solution  the  pas- 
sive iron  became  active.  The  next  important  contribution  after  the 
original  work  of  Keir  was  that  of  Schonbein17  who  showed  that  iron 
could  be  rendered  passive  by  using  it  afc?  anode  immersed  in  water 
solutions  of  oxygen  acids.  The  investigations  of  Hittorf  which 
came  about  sixty  years  later  are  extremely  interesting  since  he  found 
that  "active"  chromium  dissolves  in  the  divalent  form  while  "passive" 
chromium  dissolves  at  a  much  higher  potential  with  the  formation 
of  compounds  in  which  chromium  has  the  valence  of  six.  Periodic 
passivity  deserves  special  mention  as  it  shows  very  clearly  that,  in 
some  instances  at  least,  the  active  and  passive  states  can  be  in  a.  very 
delicate  equilibrium  with  respect  to  each  other,  for  under  the  right 
conditions  a  piece  of  iron  lying  in  nitric  acid  will  show  alternate 
periods  of  activity  and  passivity.^  There  does  not  seem  to  be  any 
regularity  to  these  pulsations  but  on  the  contrary  the  iron  may  lie 
for  a  considerable  time  and  then  suddenly  manifest  activity  almost 
with  explosive  violence  or,  on  the  other  hand,  the  change  may  start 
at  some  particular  point  and  spread  gradually  over  the  whole  surface. 
There  does  not  seem  to  be  any  real  passivity  in  the  sense  that  the  metal 
is  completely  inert  for  in  all  cases  there  is  some  loss  due  to  solution. 
This,  however,  is  not  surprising  for  even  platinum  dissolves  slightly 
in  contact  with  electrolytes  and  under  the  right  conditions  it  dissolves 
with  vigor  as  shown  by  Ruer18  who  finds  that  when  he  subjects  a  plat- 


30 

inum  pole  to  an  alternating  current  and  makes  it  simultaneously  the 
anode  of  a  direct  current  with,  small  current  density,  the  platinum  dis- 
solves extensively.  The  same  results  were  obtained  when  instead  of 
using  the  direct  current  an  oxidizing  agent  was  placed  in  the  electrolyte. 
The  experimental  work  on  passivity  has  been  very  extended  and  it 
appears  that  most  of  the  metals  exhibit  phenomena  which  can  be 
classed  under  this  head.  In  some  cases  a  visible  protecting  coating  of 
oxides  or  some  other  insoluble  material  forms,  while  at  other  times 
surface  coatings  have  not  been  demonstrated  by  very  refined  measure- 
ments of  the  reflecting  power  of  the  surfaces19.  On  the  other  hand  there 
is  some  evidence  that  passive  anodes  show  capacity  effects  which  are 
accounted  for  on  the  basis  of  a  film  of  oxide  or  oxygen.  Other  facts 
will  be  included  in  the  discussion  of  the  theories  which  follows : 


THE  FARADAY  DELICATE  EQUILIBRIUM  HYPOTHESIS. 

Schonbein  the  discoverer  of  anodic  passivity  did  not  at  first 
offer  a  theory  to  account  for  the  inactive  condition  of  iron  but  wrote 
Faraday  asking  for  an  explanation.  Faraday20  replied  saying:  "My 
strong  impression  is  that  the  iron  is  oxidized,  or  the  superficial  particles 
of  the  metal  are  in  such  relation  to  the  oxygen  of  the  electrolyte  as 
to  be  equivalent  to  an  oxidation."  He  gave  as  reasons  in  favor  of 
his  theory,  that  all  known  passivity  phenomena  were  oxidation  pro- 
cesses, that  iron  coated  with  oxide  is  insoluble  in  acids,  and  that  the 
passive  condition  disappears  on  polishing  the  metal.  Schonbein  op- 
posed  tMs  view  and  produced  evidence  which  did  not  accord  well  with 
it  and  Faraday  modified  his  statement  to  the  extent  that  he  no  longer 
maintained  that  the  coating  consisted  of  one  of  the  known  oxides,  but 
resembled  more  a  condition  of  delicate  equilibrium^  Attention  is  called 
to  this  last  clause,  for  Faraday  is  generally  credited  with  the  oxide 
film  theory  but  as  this  clearly  shows,  he  realized  that  the  situation  was 
one  of  dynamic  equilibrium. 

THE    OXIDE    THEORY. 

While  this  theory  is  clearly  a  survival  of  Faraday's  original 
idea  it  has  seemed  desirable  to  class  it  under  a  separate  head.  There  is 
no  lack  of  instances  where  fy(  anodic  solution  certain  metals  become 
coated  with  an  adhering  film  of  oxide  or  other  insoluble  material.  E. 
Muller  and  Spitzer21  showed  that  cobalt,  iron  and  nickel  on  anodic 
polarization  in  alkali  become  coated  with  oxide  and  simultaneously 
became  passive.  Haber  and  Mitland22  showed  that  in  contrast  to  the 


31 

Jbehavior  of  alkali  of  moderate  concentration  very  strong  alkali  renders 
iron  active  and  that  it  acts  by  first  dissolving  a  superficial  coat  of  oxide 
formed  when  the  iron  became  passive.  Senter15  in  his  discussion  be- 
fore the  Faraday  Society  (1914)  points  out  that  "the  weak  point  of 
such  investigation  is  that  they  are  carried  out  in  alkaline  or  neutral 
solutions,  and  therefore  cannot  afford  an  explanation  of  passivity  in 
general,  especially  in  acid  solutions."  It  is  perhaps  fitting  at  this 
point  to  raise  the  question  as  to  whether  there  is  any  explanation  of 
passivity  in  general,  or  whether,  in  particular  cases  the  actual  me- 
chanism of  passivity  depends  upon  the  conditions.  This  idea  will  be 
developed  in  more  detail  later.  Manchot23  is  one  of  the  most  recent 
supporters  of  the  oxide  theory.  His  evidence  is  based  on  the  fact 
that  silver,  iron,  copper,  cadmium,  zinc  or  lead,  plates  which  have 
been  polished  do  not  react  with  ozone  but  after  anodic  polarization  in 
NaOH  they  do  react  with  ozone  thus  proving  that  an  oxide  film  has 
been  formed  although  none  is  visible.  That  passivity  disappears  on 
standing  he  attributes  to  a  change  in  the  surface  of  the  oxide  by 
which  it  ceases  to  be  a  continuous  film  and  contracts  to  granular  par- 
ticles thus  exposing  the  free  metal  again. 

THE    VALENCY    THEORY    OF    PASSIVITY. 

This  theory  has  had  little  support  although  it  is  particularly 
in  accord  with  the  behavior  of  chromium  and  molybdenum,  which 
when  passive  dissolves  with  higher  valency  than  when  active.  However, 
it  has  not  been  shown  that  the  solution  of  these  metals  in  a  higher 
state  of  oxidation  when  passive  is  not  due  to  a  secondary  reaction. 
The  theory  is  based  on  Kruger's25  assumption  that  the  modifications 
of  a  metal  with  different  valencies  are  all  present  in  the  solid  metal 
in  proportions  depending  on  the  conditions,  as  temperature,  etc.  This 
theory  of  Kruger  and  Finkelstein  then  accounts  for  passivity  on  the 
actual  condition  of  the  metal  itself.  While  interesting  it  has  not 
been  productive  although  attention  should  be  called  here  to  the  fact 
that  chromium  becomes  passive  in  halogen  solutions.  The  most  re- 
cent work  along  this  line  is  that  of  W.  J.  Muller,  191024  who  finds 
that  in  KOH  solutions  thallium  goes  into  solution  with  valency  vary- 
ing from,  one  to  three. 

THE  OXYGEN  CHARGE  HYPOTHESIS. 

Fredenhagen26,  who  first  suggested  this  hypothesis,  considered 
that  anodic  oxygen  depresses  the  velocity  of  the  reaction  of  the  metal 
with  itself  to  such  an  extent  that  the  anode  covers  itself  completely 


32 

with  a  film  of  oxygen  or  an  alloy  of  oxygen  and  the  metal,  thus  in- 
ducing passivity.  Considering  first  the  idea  of  an  oxygen  film  en- 
velope covering  and  protecting  the  metal  surface,  this  view  taken  in 
the  extreme,  would  mean,  just  as  the  oxide  film,  that  there  is  a  two 
phase  system.  On  the  other  hand,  the  idea  of  an  oxygen  alloy  in- 
volves a  one  phase  system  of  continuously  variable  composition, 
and  a  correspondingly  variable  electrolytic  solution  pressure.  In 
addition  it  may  be  stated  that  it  is  quite  plausible  that  should  they 
exist  either  one  or  both  of  the  above  systems  might  be  complicated  by 
•the  formation  of  an  oxide,  which  would  exist  in  equilibrium.  Such  a 
state  of  affairs  would  remind  one  of  Faraday's  view  that  the  con- 
dition is  one  of  "very  delicate  equilibrium." 

HYDROGEN  AOTIVIFICATION    HYPOTHESIS. 

That  all  metals  in  the  pure  condition  are  passive  is  suggested 
by  Foerster27,  and  that  they  become  active  under  the  influence  of 
catalytic  agents.  Hydrogen  is  suggested  as  the  most  important  of 
these  catalysts.  E.  Grave28  disagrees  with  him  to  the  extent  of  say- 
ing that  hydrogen  ions  are  the  activating  agents.  Heating  metals  in 
nitrogen,  vacuum,  etc.,  and  then  testing  the  passivity  has  not  yet  set- 
tled the  question  since  different  investigators  obtain  results  which  do 
not  agree.  Of  course  it  is  probably  not  true  that  heating  will  com- 
pletely free  the  metals  fr,pm  dissolved  gases,  and  this  iri*indicated  by 
the  fact  that  cathodic  bombardment  of  iron  will  cause  gas  to  be  evolved 
for  an  indefinite  period  of  time.  The  work  of  Grave  above  referred 
to  has  shown  strong  evidence  that  a  metal  can  be  rendered  active  by 
the  diffusion  of  hydrogen  ions.  On  the  whole  it  has  been  fairly  well 
established  that  gases  may  play  an  important  part  as  catalysts;  yet 
our  scanty  knowledge  of  absorption  or  occlusion  of  gases  by  metals 
limit  interpretation  of  results.  The  difficulties  encountered  shows 
that  we  very  rarely  work  with  absolutely  pure  materials  and,  since 
catalyzers  are  effective  even  though  they  may  be  present  in  the  mer- 
est traces,  our  system  may  be  complicated  by  unknown  factors.  It 
is  most  certainly  true  that  different  samples  of  iron  show  a  marked 
difference  of  behavior. 

The  details  of  the  hydrogen  activification  hypothesis  as  worked 
out  by  Grave15  are  not  as  illuminating  as  one  might  hope.  He  says, 
"that  just  as  water  and  other  liquids  do  not  boil  even  when  their  vapor 
pressure  is  equal  to  the  superincumbent  pressure,  unless  a  catalyst, 
such  as  air  is  present,  so  metals  which  can  be  passified  dissolve  rapidly 
only  in  the  pressure  of  a  catalyst.  It  might  be  supposed  that  hydrogen, 


33 

like   air   in   boiling,   forms   a   nucleus    around   which   the   metal   ions 
might  collect." 

PRIMARY    ANODIC    DISCHARGE    HYPOTHESIS. 

Sackur29  suggests  that  the  solution  of  a  metal  as  an  anode  is 
not  due  to  the  passage  of  ions  of  the  metal  into  the  solution  but  rather 
to  the  primary  discharge  of  the  ions  which  then  react  directly  with 
the  metal  itself.  On  the  basis  of  this  idea  he  gave  a  rather  com- 
plicated explanation  which  has  not  proved  useful ;  but  the  idea  of 
primary  anodic  discharge  is  one  that  promises  to  play  an  important 
part  in  future  passivity  considerations,  as  well  as  to  find  a  place  in 
the  other  fields  of  electro-chemistry. 

THE    ION    HYDRATION    HYPOTHESIS    OF    LE    BLANC. 

Le  Blanc30  was  the  first  to  put  forward  the  idea  that  the  pas- 
sive state  is  due  to  a  change  in  the  velocity  of  ionization.  How  this 
change  was  brought  about  he  did  not  at  first  attempt  to  say.  He  has 
now  extended  his  views  and  accounts  for  the  retardation  on  the  basis 
of  hydration  of  the  ions.  He  says  the  idea  that  ions  of  the  metal  are 
not  formed  primarily  may  be  justified,  but  he  contends  that  this  as- 
sumption is  not  necessary.  He  prefers  to  think  that  the  ions  of  the 
metal  are  primarily  formed  and  practically  with  infinite  velocity  but 
that  they  tend  to  become  hydrated  at  the  surface  of  the  electrode.  The 
magnitude  of  the  potential  difference  between  the  metal  and  the 
electrolyte  would  then  depend  upon  the  concentration  of  the  non- 
hydrated,  free  ions,  and  this  latter  again  upon  the  velocity  of  hydra- 
tion. Hb  is  thus  able  to  account  for  anodic  and  cathodic  polarization. 
With  passive  metals  ionic  hydration  occurs  so  slowly  at  the  anode 
that  the  concentration  of  free  ions,  and  therefore  the  potential  dif- 
ference, becomes  so  great  that  visible  separation  of  the  negative 
radical  or  of  oxygen  results. 

The  above  is  not  a  complete  catalogue  of  the  hypotheses  that 
have  been  offered  for  passivity,  but  the  most  productive  in  stimulating 
research.  No  one  of  them  will  completely  correlate  and  explain  all 
of  the  wide  range  of  facts  bearing  on  the  question.  Each  has  its  ad- 
vantages as  well  as  its  limitations. 

In  view  of  the  diversity  of  opinion  and  wide  range  it  seems, 
as  already  suggested,  that  there  is  no  one  general  explanation  of  pas- 
sivity but  rather  that  in  each  individual  case  the  loss  of  activity  is 
dependent  upon  the  conditions,  and  that  the  explanation  of  the  me- 
chanism of  the  reaction  will  not  come  at  one  step  by  the  announce- 


34 

ment  of  an  all  embracing  hypothesis  but  that  each  individual  case  must 
be  worked  out  independently.       Iron  anodes  in  sulphuric  acid  behave 
differently  depending  upon  the  circumstances.      This  has  been  pointed 
out  by  Schulze15  who  says  that  "in  dilute  sulphuric  acid  iron  shows- 
passivity;  in  concentrated  acid,  valve  action;  in  intermediate  concen- 
tration an  unstable  valve  action  succeeded  by  passivity."     The  nature 
of  valve  action   is  made  clear  by  the  behavior  of   aluminium,   when 
used  as  an  anode  in  a  suitable  electrolyte  a  film  of  aluminium  oxide 
is  formed  and  oxygen  is  evolved.      The  film  is  at  first  so  thin  that 
it  is  invisible  but  gradually  builds  up  to  a  light  gray  coating  pierced 
by  very  numerous  holes  of  microscopic  size.      This  film  absorbs  about 
5%  of  the  current,  the  rest  being  used  in  the  formation  of  the  oxygen 
which  escapes.      The  work  and  conclusion  of  Schulze,  although  bear- 
ing largely  on  valve  action,   throw  interesting  and  valuable  light  on 
the  passivity  problem.      The  objection  is  often  raised  that,  as  in  the 
case  of  iron,  it  is  difficult  to  understand  how  oxides  or  hydroxides  can 
exist  in  the  presence  of  an  acid  electrolyte.     Let  us  consider  the  case 
of  a  metal  used  as  an  anode  when  it  is  readily  seen  that  there  is  no 
objection  to  the  assumption  that  the  surface  of  the  metal  is  in  con- 
tact with  a  neutral  or  alkaline  medium,  for  since  the  anode  is  posi- 
tively charged  the  hydrogen   ions  which   bear   a  charge  of  like  sign 
would  be  repelled,  and  their  concentration  materially  reduced  in  the 
zone  of  contact  between  the  electrolyte  and  anode.     On  the  other  hand 
negative  ions  would  be  attracted  and  increase  the  possibility  of  primary 
anionic  discharge.      Thus  the  relations  of  iron  to  sulphuric  acid  are 
very  complex,  and  it  will  be  sufficient  to  outline  the  conclusions  ar- 
rived at  with  respect  to  the  mechanism  of  the  establishment  of  pas- 
sivity in  iron  when  used  as  an  anode  in  fifth  normal  sulphuric  acid, 
and  under  the  specific  conditions  which  have  been  maintained  through- 
out the  experimental  work.      There  is  little  doubt  but  that  we  are 
concerned   with   surface   conditions   in   our   study   of   passivity.      The 
potential  curve  shows  that  what  occurs  when  passivity  is  established 
is   a   two   process   change.      The   external   current   curve   agrees   with 
this  and  indicates  that  after  the  application  of  the  current  there  is 
almost  at  once  an  increase  in  the  resistance  at  the  surface  of  the  elec- 
trode and  that  after  a  time  there  is  another  sudden  and  greater  in- 
crease in  this  resistance. 

Let  us  now  consider  a  reaction  mechanism  that  would  account 
for  these  changes.  The  ordinary  facts  of  electrode  potentials  show 
that  metals  can  and  do  form  ions  directly  so  that  even  with  our  iron 
anode  it  is  very  probable  that  there  is  in  equilibrium  between  the 
metal  and  its  ions  Fe  metal  (Fe^).  There  is  also*  evidence  that  the 


35 


(Ions) 


SO, 
r/ons) 

\ 
(FeSOj 


ions) 


r&dfc-ats) 


to 


\ 


Fig. 


(0)&+omic  Oxygen  -\-  HySC^ 
f  | 

Oa    In  aolvt-ion 
0B  GCH, 


anions  may  be  attracted  to  the  surface  of  the  anode  and  discharge 
directly  on  the  iron  to  form  free  (SO4)  radicals.  These  latter  may 
react  directly  with  the  metal  to  form  ferrous  sulphate  or  may  react 
with  water  to  form  oxygen  atoms,  (SO*)  +  HK)  =  H»SO*  +  (O).  The 
oxygen  atoms  will  react  with  the  iron  to  form  insoluble  ferrous  oxide 
Fe  +  (O)  —  FeO,  or  form  molecular  oxygen  with  the  corresponding 
equilibria:  (O)  +  (O)  =  O*  in  solution  --  gas  O2.  The  principal 
equilibrium  conditions  are  graphically  illustrated  in  Fig.  9. 

At  the  moment  that  the 
iron  becomes  an  anode  the 
most  of  the  current  is  car- 
ried by  means  of  iron  ions 
entering  the  solution.  But 
some  anions  are  discharged 
directly  on  the  surface  of  the 
iron  and  thus  carry  part  of 
the  current.  These  dis- 
charged ions  can  react  with 
the  iron  to  form  ferrous  sul- 
phate, or  with  the  water ^  to 
form  atomic  oxygen  which  in  turn  can  form  ferrous  oxide  or  oxygen 
molecules.  In  the  latter  case  the  solution  would  become  saturated  with 
oxygen  and  the  gas  would  escape.  But  the  formation  of  bubbles  against 
an  external  pressure  is  a  process  that  requires  energy  and  it  seems  that 
so  long  as  the  free  iron  is  readily  available  it  requires  less  work  to 
form  the  very  insoluble  ferrous  oxide.  However,  the  more  ferrous 
oxide  that  is  formed  the  less  free  iron  is  available  and  the  more  sul- 
phate ions  are  primarily  discharged  so  that  the  concentration,  of  oxy- 
gen atoms  would  be  so  increased  that  the  equilibrium  would  be  changed 
to  the  direction  of  the  formation  of  oxygen  gas.  On  saturation,  if 
the  intensity  of  the  reaction  is  sufficient,  gas  will  form  and  the  elec- 
trode become  covered  with  a  film  of  oxygen,  the  high  resistance  of 
which  accounts  for  the  last  final  drop  in  the  potential  and  current 
curves.  Due  to  the  surface  tension  this  formation  of  the  gas  film  will 
be  quick  and  complete. 

This  hypothesis  fits  the  facts  which  are  characteristic  of  the 
behavior  of  passive  iron  anodes  under  the  conditions  studied.  Particu- 
lar attention  is  called  to  the  strong  confirmatory  evidence  given  by 
the  fact,  established  in  this  research,  that  dissolved  oxygen  increases 
the  speed  with  which*  passivity  is  established.  (See  Table  VII.)  With 
the  high  oxygen  pressures  the  equilibrium  conditions  are  made  more 
favorable  for  the  formation  of  the  ferrous  oxide  and  for  the  formation 


36 

of  the  oxide  film.  This  is  shown  by  a  glance  at  the  graphical  repre- 
sentation given  above.  The  fact  that  stirring  retards  the  establishment 
of  passivity  is  in  perfect  accord  for  it  certainly  would  tend  to  tear  off 
the  film,  of  oxide  thus  keeping  fresh  iron  constantly  exposed.  How- 
ever, after  the  passivity  has  been  once  established  the  tendency  of  stir- 
ring to  destroy  the  condition  would  be  reduced  to  a  minimum  by  the 
influence  of  surface  tension  to  keep  the  gas  film  intact.  The  reestab- 
lishment  of  activity  occurs  when,  after  the  external  current  has  been 
stopped,  the  gas  film  becomes  ruptured  in  any  way.  Then  the  iron  and 
oxide  layer  are  in  relation  to  each  other  very  much  like  that  of  a  short 
circuited  cell  and  the  activification  occurs  almost  at  once. 

The  retarding  influence  of  higher  temperatures  offers  no  difficulty. 
The  fact  that  very  low  current  densities  do  not  passivify  is  not  surpris- 
ing in  view  of  the  equilibrium  conditions  which  have  to  be  established. 
It  is  not  my  purpose  to  review  all  of  the  facts  with  the  idea  of  showing 
how  they  accord  with  the  explanation  offered  here. 

SUMMARY. 

The  previously  described  retardation  of  the  establishment  of 
anodic  passivity  by  the  magnetic  field  was  shown  to  be  due  to  a  stirring 
produced  in  the  electrolyte  when  the  cell  was  in  the  magnetic  field. 
Exactly  analogous  retardations  were  produced  by  rotating  the  anode 
and  the  effect  being  greater  with  greater  agitation.  It  was  shown  that 
the  concentration  of  dissolved  oxygen  in  the  electrolyte  has  a  very  great 
influence  in  the  direction  of  hastening  the  processes  of  establishing 
anodic  passivity. 

Interesting  results  on  the  relation  of  polarization  potentials  to 
external  pressure  were  shown. 

The  curves  showing  the  variation  with  time  of  electrode  potential 
and  of  polarizing  current,  during  the  course  of  passivification,  indicate 
that  the  two  processes  are  involved. 

A  summary  of  the  facts  and  theories  of  passivity  is  given  and  it 
is  suggested  that  there  is  no  general  explanation  of  passivity,  but  that 
development  must  come  through  an  understanding  of  the  mechanism 
of  individual  cases.  However,  Faraday  approached  an  explanation 
when  he  said  that  passivity  is  a.  condition  of  very  'delicate  equilibrium. 

A  hypothetical  explanation  is  given  for  the  particular  case  studied. 

As  a  final  word  the  author  wishes  to  express  his  great  indebtedness 
to  Professor  Horace  G.  Byers  for  sustained  inspiration,  suggestion  and 
counsel  throughout  the  course  of  this  research.  Gratitude  is  expressed 
to  Dr.  Harlan  L,  Trumbull  for  helpful  advice,  and  to  Professor  Henry 
L.  Brakel  for  the  use  of  apparatus  from  his  electrical  laboratory. 


37 

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2  Byers,   Am.   Chem.   Soc.,  30,    1718,    (1908). 

3  Heathcote,  J.  Soc.  Chem.  Ind.,   26,   899,    (1907). 

4  Byers  &  Darrin,  J.  Am.  Chem.  Soc.,  32,  750,   (1910). 

5  Byers  &  Morgan,  J.  Am.  Soc.,  35,  1757,  (1911). 

6  Byers  &  Langdon,  J.  Am.  Chem.  Soc.,  35,  759,    (1913). 

7  Fredenhagen,  Z.  physik.  Chem.,  43,  1;   63,  1. 

8  Graves,  Z.  physik  Chem.,  77,  513. 

9  Hurnuzecum,  Elect.  Rev.,  42,  322. 

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11  Frary,  J.  Am.  Chem.  Soc.,  29,  1592. 

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13  Urbasch,  Z.  Electrochem,   7,   114;    7,  527. 

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23  Manchot.   Ber.   42,   4942. 

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28  E.  Grave,  Zeit.,  physikal.  chem.,  (1911),  77,  513. 

27  Foerster,  Abhandlunge  der  Bunsen  Ges.,  No.  2,   (1902). 

29  Sackur,  Zeit.  Electro  Chem.,    (1908),  14,  607. 

30  Le  Blanc,  Lehrbuch  der  Electrochemie. 


AUTOBIOGRAPHY. 

The  writer  was  born  at  Leadville,  Colorado,  and  received  his 
early  education  in  the  public  schools  of  Illinois.  He  received  the 
Bachelor  of  Science  degree  at  Northwestern  University  in  1911.  The 
two  following  years  were  spent  as  a  graduate  assistant  at  the  University 
of  Washington,  the  degree  of  Master  of  Arts  being  granted  in  June, 
1913.  Through  the  aid  of  Loretta  Denny  fellowships  work  was  con- 
tinued in  the  same  institution  during  the  years  1913-1915. 


